This invention pertains to a process for providing surface layers of copper or copper alloyed with zinc on die castings of zinc or zinc alloys, particularly but not exclusively as a pretreatment before conventional nickel plating, which may be followed by decorative chromium plating.
As a pretreatment of die castings of zinc alloys before electroless nickel plating, which may be followed by decorative chromium plating, it is conventional for a copper strike to be applied in an electroplating bath containing copper cyanide, as explained in W. H. Safranek, "Plating on Zinc Alloy Die Castings," Chapter 26 in F. A. Lowenheim, Editor, Modern Electroplating (Wiley-Interscience, 3rd Edition, 1974) at pages 613-615, and as also explained in "Decorative Chromium Plating," Volume 2, Metals Handbook (American Society for Metals, 8th Edition, 1964) at pages 458-460. Cf. U.S. Pat. No. 3,664,993 to R. J. Clauss.
Various immersion processes, electroless processes, and autocatalytic processes also are known for plating of copper on various metals, which include zinc in some processes. Some examples of such processes are discussed hereinbelow. In connection with such processes, it is to be noted that in the electrochemical series (standard electrode potentials of metals, as related to that of hydrogen) zinc and copper differ markedly. See Table 2 in F. A. Lowenheim, op. cit., at page 776.
U.S. Pat. No. 43,557 to F. Weil discloses that a coat of copper may be deposited on zinc immersed in an alkaline solution of a copper salt and sodium-potassium tartrate, or of a copper salt and ammonia only. Sodium-potassium tartrate, which is a double salt, is known commonly as Rochelle salt.
U.S. Pat. No. 3,716,462 to D. P. Jensen discloses that copper may be plated on a body of zinc or zinc alloy in an electroless bath containing a soluble salt of copper, a complexing agent, and a reducing agent, at a pH ranging from about 5.0 to about 13.0, and that sodium hydroxide may be added if an alkaline bath is to be employed.
U.S. Pat. No. 3,033,703 to F. W. Schneble, Jr. et al. discloses a bath for electroless copper plating on various metals at a pH of at least 11.0, wherein the bath contains a copper salt, a complexing agent, and a reducing agent. Although cupric oxide and cuprous oxides ordinarily are not considered salts, both are among the "salts" listed as suitable in U.S. Pat. No. 3,033,703.
U.S. Pat. No. 4,036,651 to J. A. Weiner discloses a process of related interest, wherein a pH of 11-13.3 is specified for a bath for electroless copper plating.
U.S. Pat. No. 3,046,159 to R. R. Brookshire discloses a process for electroless copper plating on various metals in an acidic bath, wherein a complexing agent maintains a small amount of copper in solution while a large amount of copper remains out of solution in copper oxide, which is insoluble in water, and wherein a reducing agent reduces copper from solution.
As additional background, it is known that cupric oxide and cuprous oxide are insoluble in water but soluble in sodium hydroxide, whereupon little of either oxide can be dissolved in dilute aqueous solutions of sodium hydroxide.